How to Calculate the Level Measurement in a Pressurised Tank

We use two pressure measurements and some maths and differential pressure to calculate level, flow, interface, and even density. Its versatility and diverse outputs make it far more valuable in a wide range of industries. This article focuses solely on how differential pressure can be used to calculate level measurements in pressurized tanks. The advantages, limitations, and common industrial applications of differential pressure will be discussed.

How to use a differential pressure transmitter to calculate the level measurement in a pressurised tank

1. How to work with differential pressure

These opposite pressure measurements are pushed on opposite sides of the double-sided diaphragm, and the resulting size is the pressure difference or differential pressure between the two. Conventional differential pressures cover a wide range of applications and can detect pressure differences of just a few millibars.

The pressure transmitter is connected to the vessel in two ways – a pulse line or a capillary line. Pulse lines are hard lines, it allows the fluid or gas in the process to come into direct contact with the diaphragm in the measuring device, and these lines become part of the process. Connected to these external diaphragms are flexible, oil-filled, armor-coated lines. In addition, remote seals separate the intelligent pressure transmitter from potentially harmful process parameters.


These two arrangements both measure pressure. However, the pressure measurement at the bottom of the tank or vessel measures the overall pressure generated by the fluid and the vapor space above it. In contrast, the pressure measurement at the top only considers that head or static pressure. This arrangement allows the static pressure to be eliminated from the overall measurement, thus allowing the pressure generated by the fluid to be retained and thus the level to be inferred.

2. Common applications of differential pressure

We use pressure tanks for several reasons, such as providing a constant output, eliminating foam, providing a barrier for corrosive materials, and liquefying gases for storage. However, the differential pressure transmitter will only measure the difference between the static pressure and the total pressure in all these cases. Therefore, when we calculate the product level, we need some mathematical operations.

The standard hydrostatic pressure formula is made up of three variables: pressure, density, and height. When the sensor measures pressure, thickness is entered by the customer as a constant, and measurement is the product height. For us to make this formula work, density is critical and must remain reasonably consistent. Therefore, we use known values of density and pressure, and the pressure sensor electronics can accurately and reliably calculate the level based on the differential pressure.

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3. Limitations of differential pressure

Differential pressure is one of the most effective levels measurement methods, but it does have its drawbacks. Our installation requires the process to be stopped and then we have to empty the vessel in which the measurement is being made. This can be an expensive or time-consuming process, especially if multiple containers require this type of measurement.

In addition, we use differential pressure measurement with its pulse lines and capillary lines, which are susceptible to external influences. There is a risk of these lines being crushed if they are in areas of high footfall, and if they are outside climate-controlled areas, the lines can become hot or cold, leading to changes in material density and level measurement errors. Fortunately, there is a solution to these common problems.

4. Electronic Differential Pressure

If we want to do things more efficiently, we have to create electronic differential pressure using an innovative combination of software and hardware. The system uses two sound pressure transducers connected directly to the vessel or tank via a thin cable. This setup does not require a traditional double-sided transducer, and we do not need pulses or capillary lines; thus, we are making installation and maintenance much more accessible.

The electronic differential pressure principle is the same as for a single sensor, and we use two different pressure outputs to determine the differential pressure. The method requires only more mathematics to achieve a horizontal production. A pressure transmitter is a primary sensor that provides an overall measurement of the product and vapor space. In contrast, a secondary sensor provides an odd pressure measurement of the vapor space for the primary sensor. The primary transmitter uses a simple calculation to subtract the two, essentially eliminating the vapor space from the equation, and then performs some maths to provide the level output using the hydrostatic equation.

How to use a differential pressure transmitter to calculate the level measurement in a pressurised tank (2)

The electronic differential pressure reduces the need for pulsed or capillary lines, thus eliminating the sensitivity to external influences that cause measurement errors. It also opens up the possibility of better diaphragm options, such as ceramic, which is abrasion-resistant and better suited to withstand harsh environments—aIn addition, avoiding the use of pulsed or capillary lines and using measuring cells that are ten times tougher than stainless steel can extend accurate level measurement with little or no maintenance.

5. Conclusion

The versatility of the measurement is matched only by the sensor, which has customizable options. It includes multiple housing positions, allowing the sensor to reach situations where previous differential pressure sensors did not fit. Furthermore, the electronic differential performs all these tasks and provides the processor with additional measurement options. All this proves that differential pressure is and will continue to prove itself.


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